1. Field of the Invention
The present invention relates generally to detecting optical signals and, more particularly, to detecting multiple optical wavelengths with optical supergratings.
2. Prior Art
Gratings are optical devices used to achieve wavelength-dependent characteristics by means of optical interference effects. These wavelength-dependent optical characteristics can, for instance, serve to'reflect light of a specific wavelength while transmitting or refracting light at all other wavelengths. Such characteristics are useful in a wide range of situations, including the extraction of individual wavelength-channels in Wavelength Division Multiplexed (WDM) optical communication systems, or providing wavelength-specific feedback for tunable or multi-wavelength semiconductor lasers. Gratings are usually implemented by modulating (varying) the effective index of refraction of a waveguiding structure. These changes in index of refraction cause incident light wavelengths to be reflected or refracted: in the case of an abrupt interface between two index values, light incident directly on the interface is reflected according to the well known Fresnel reflection law.
The term “multi-wavelength grating” generally refers to a grating that is capable of exhibiting optical characteristics at a number of wavelengths. For example, a multi-wavelength grating can be a grating that reflects light at several select wavelengths (which can correspond to specific optical communication channels), yet is transparent to light at other wavelengths. In some situations, however, there is a need to set the optical characteristics for a continuous range of wavelengths, rather than at specific wavelength values. For example, when trying to compensate for the unevenness of optical gain profiles in laser cavities and optical amplifiers by means of an optical grating. However, achieving this requirement for a continuous range of wavelengths is difficult to meet with traditional grating technologies.
Similarly, a range of optical wavelengths may be used where many communication channels are encoded into a single optical cable by utilizing different wavelengths of light; more commonly known as Wavelength Division Multiplexing (WDM) technology. Periodic gratings are often used to separate or process these channels. However, periodic grating technologies process one wavelength, forcing devices intended to process multiple wavelengths to employ multiple single-wavelength periodic gratings. This is not an attractive solution because, on top of the additional losses that each grating creates, even a single grating occupies a considerable amount of space by today's standards of integration and miniaturization. It is thus desired to have a single device capable of processing several wavelengths in a space-efficient manner.
In the realm of semiconductor lasers, the output wavelength of semiconductor lasers is largely determined by the presence of “feedback elements” around, or inside the laser gain section, which act to reflect light at the desired wavelength back into the laser. For multi-wavelength operation, multi-wavelength feedback is needed. Again, single-wavelength grating technology can only address this demand with a cascade of simple gratings, leading to the same (if not more notable) loss and space problems mentioned above.
One such single-wavelength grating device, is a Bragg Grating. The Bragg Grating consists of a periodic variation in refractive index and acts as a reflector for a single wavelength of light related to the periodicity (known as pitch, A) of the index pattern; and is frequently used in both semiconductor systems and fiber-optic systems. In practice, however, the Bragg Grating can actually reflect at several wavelengths, corresponding to overtones of its fundamental pitch. However, these higher-order wavelengths tend to be at quite different spectral regions than the fundamental wavelength, thus making the Bragg Grating less than useful as a multi-wavelength reflector. Moreover, these higher-order wavelengths cannot be tuned independently of one another.
Other multi-wavelength grating technologies include: analog superimposed gratings, Sampled Gratings (SG), Super-Structure Gratings (SSG) and Binary Supergratings (BSG).
Analog superimposed gratings are a generalization of the Bragg Grating and are rooted in a principle of superposition: a grating profile consisting of the sum of the index profiles of single-wavelength gratings reflects at all of its constituent wavelengths. Such a grating relies on an analog index variation, that is, a refractive index that changes continuously along the grating length (FIG. 30). However, it is difficult to inscribe strong analog gratings using the well known photorefractive effect, since the change of index under illumination varies non-linearly, and generally saturates with stronger exposures. Likewise, rendering surface-relief analog gratings (a typical embodiment for semiconductors) is made impractical by the difficulty of reproducibly etching analog features into a surface. The latter difficulty brought about the introduction of binary gratings, i.e., gratings that rely only on two refractive index values corresponding to the material being etched or not etched, illuminated or not illuminated.
Two representations of multi-wavelength binary gratings are sampled gratings (SG) and superstructure gratings (SSG). The SG is constructed with alternating sections of grating and grating-free regions of the waveguide. The alternating sections produce diffraction spectra having multiple reflectance peaks contained within a (typically) symmetric envelope. The SG is intrinsically limited in the flexibility in the location and relative strength of reflectance peaks, and, because of the large fraction of grating-free space, is also spatially inefficient. The SG is therefore particularly unsuitable where a short grating is required or where waveguide losses are high.
With the super-structure grating (SSG), the grating period is chirped by finely varying the grating pitch, which corresponds to the length of one tooth-groove cycle. This can also be thought of as a sequence of finely tuned phase shifts; common phase profiles include linear and quadratic chirp. Such an implementation in principle allows arbitrary peak positions and relative heights, but only at the expense of extremely high resolution, corresponding to a very small fraction of the size of the grating teeth themselves.
Prior art regarding binary superimposed grating synthesis is presented in Ivan A. Avrutsky, Dave S. Ellis, Alex Tager, Hanan Anis, and Jimmy M. Xu, “Design of widely tunable semiconductor lasers and the concept of Binary Superimposed Gratings (BSG's),” IEEE J. Quantum Electron., vol. 34, pp. 729-740, 1998.
Other methods in the prior art address the synthesis of “multi-peak” gratings—i.e., gratings characterized by reflectance at several “peaks”, which can be controlled in their position and strength. In these methods, a grating engineer begins with a set of sinusoids, each sinusoid corresponding to a single reflectance peak and weighted according to that peak's desired relative strength. These peaks are added together (i.e. superimposed; hence the BSG is known as a superimposed grating) to produce an “analog profile”. This profile is then digitally quantized by a simple threshold method.
For example, if the analog profile value is positive (above a pre-selected reference) then the corresponding BSG segment is a high or binary 1 index value; if it is negative, the corresponding BSG segment is a low or binary zero index value.
However, this approach is inadequate in at least two areas: firstly, the threshold quantization process introduces intermodulation, which largely limits the applicability of BSGs synthesized in this manner to active applications (laser feedback elements and the like). Secondly, this synthesis procedure is limited to multi-peak gratings, and offers little or no control over the individual peak shape. For example, it is entirely incapable of generating flattop channels, as desired by some communication applications, or of generating the near-arbitrary reflectance spectra demanded by some gain-compensation and dispersion-compensation methods.
Other methods for BSG synthesis include trial-and-error methods that are most often computationally difficult and inefficient.
Therefore, it is desirable to provide a method and apparatus for overcoming the disadvantages noted above in designing and synthesizing supergratings for detecting optical wavelengths.